A nitric acid is produced by catalytic oxidation or ammonia with air. The air is compressed to between 4 and 14 Bar, is mixed with ammonia vapors, and is passed to a catalytic reactor. If the operating pressure is lower than 6 Bar, it is possible to evaporate the liquid ammonia for refrigeration purposes. However, at a low pressure, the rates of reactions of nitric oxides conversion and absorption are low. Therefore, the reactor effluent gas is cooled and is further compressed to a higher pressure. When the catalytic oxidation is done at pressures around 10 Bar, the ammonia vapors are obtained by heating liquid ammonia with water or low pressure steam, and no refrigeration is produced at all.
Nitric acid plants are frequently located next to ammonia synthesis plants which need large amounts of refrigeration. For example, in a 1,000 tons a day ammonia plant, the power needed for a refrigeration compressor is between 1,500 and 4,000 kW. At an early stage of ammonia synthesis technology, the ammonia was recovered from the synthesis loop by absorption in water or a lean ammonia solution. The thus obtained ammonia solution could be used in the nitric acid production process. However, this process was abandoned because the synthesis loop after the absorption contained water which was undesirable in the synthesis. If air would be used to strip ammonia from the solution, there would be another problem, namely, some carbon dioxide from the air would remain in the water solution. In the ammonia synthesis, carbon dioxide is even less desirable than water.
A closed loop absorption refrigeration generally includes the steps of:
(a) evaporating liquid ammonia;
(b) absorbing an ammonia vapor in a lean solution to obtain a rich solution. The heat of absorption is transferred to a cooling water;
(c) pumping rich solution to a higher pressure, the ammonia being stripped from the solution in a distillation column and reboiler to obtain a lean solution;
(d) condensing the overhead ammonia vapor and recycling the liquid ammonia to the step (a), while the lean solution is recycled to the step (b).
The reboiler in step (c) is operated at temperatures around 160.degree. C. and a substantial amount of heat is needed to drive out the ammonia from the solution. For each kilogramm of evaporated ammonia about 1,000 Joules of refrigeration is obtained in step (a) and about 1,500 Joules of heat is needed in step (c). The process is clearly highly uneconomical, when compared to the process of the invention.